Abstract:
Testicular samples were collected from African sideneck turtles (Pelusios castaneus) at the peak of
spermiogenesis in order to describe spermatid acrosomal vesicle formation and nuclear morphogenesis.
Acrosomal vesicle formation commences with a Golgi transport vesicle attaching to a round spermatid,
followed by the emergence of an acrosome granule. This is followed by the development of the subacrosomal
space, which becomes enlarged as nuclear elongation and condensation continue. The round
spermatid elongates and the emerging elongating spermatid successively becomes surrounded by circular,
longitudinal and slanting microtubules of the manchette. The acrosomal vesicle becomes visible with
an acrosome granule resting on the base of the electron dense material. Acrosomal vesicle morphogenesis
in the African sideneck turtle results in a highly compartmentalized acrosome divisible into the acrosomal
cortex and medulla. The future position of the flagellum starts to develop, being encircled by mitochondria
while the distal centriole becomes obvious and the emerging flagellum grossly divisible into the
connecting piece, midpiece, principal piece and endpiece. Although acrosomal vesicle formation and
nuclear morphogenesis during spermiogenesis in the turtle are consistent with other reptilian species,
a few differences were observed. The major difference observed was the formation of a single acrosome
granule, which manifests prior to the attachment of the acrosomal vesicle to spermatid nucleus. The
other differences observed were the emergence of two endonuclear canals in the elongating spermatid
and the presence of slanting microtubules of the manchette. The observed developmental variations
are expected to be valuable in future phylogenetic studies and potentially serve to test certain hypotheses
concerning the reproductive status of turtle species. Findings from this study add to the growing database
of spermatid morphology in turtles, thereby providing insights into variations in mature sperm morphology
in the species.
